Musnad Ahmad 30:2057

Mu'awiyah narrated to us, he said: Za'idah narrated to us, from Muhammad—meaning Ibn 'Amr—from Abu Salamah, from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), who said: "Were it not that I would make it difficult for the believers—or for my Ummah—I would have commanded them to use the siwak at every prayer."